Есть класс DataContractJsonSerializer .
Deserialize:
DataContractJsonSerializer ser = new DataContractJsonSerializer(typeof(MyObject));
Stream s = new MemoryStream(System.Text.Encoding.UTF8.GetBytes(json_string));
MyObject obj = ser.ReadObject(s) as MyObject;
Сериализация:
DataContractJsonSerializer ser = new DataContractJsonSerializer(typeof(MyObject));
Stream s = new MemoryStream();
MyObject obj = new MyObject { .. set properties .. };
ser.WriteObject(s, obj);
s.Seek( SeekOrigin.Begin );
var reader = new StreamReader(s);
string json_string = reader.ReadToEnd();